Electron discharge device



NOV. 30, 1937. SALZBERG 2,100,723

ELECTRON DISCHARGE DEV-ICE Original Filed Sept. 22, 1955 INVENTOR,BERNARD SALZBERG ATTORNEY.

Patented Nov. 30, 1937 ELECTRON DISCHARGE DEVICE Bernard Salzberg, EastOrange, N. .L, assignor, by mesne assignments, to Radio Corporation ofAmerica, a corporation of Delaware Original application September 22,1933, Serial No. 690,504. Divided and this application October 4, 1935,Serial No. 43,465

3 Claims.

My invention relates to improvements in electron discharge devices, andits principal object is to provide an improved electron dischargedevice, particularly of the type having three or more 5 electrodes anduseful as an amplifier, which has more desirable characteristics andparticularly a more nearly constant amplification factor than such tubesconstructed in the usual way.

The present application is a division of my copending application SerialNumber 690,504, filed September 22, 1933 and assigned to the sameassignee as the present application.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims, but theinvention itself will best be understood by reference to the followingdescription taken in connection with the accompanying drawing in which:

Figure l is a longitudinal section of an electron discharge deviceembodying my invention;

Figure 2 is a horizontal cross section of the electron discharge deviceshown in Figure 1 taken along the line 2-2 of Figure 1;

Figure 3 is a horizontal cross section of a modified form of theelectron discharge device shown in Figure 1;

Figure 4 is a horizontal cross section of another modified iorm of theelectron discharge device of Figure 1;

Figures 5 and 6 are diagrams for graphically comparing certaincharacteristics of an electron discharge device embodying my inventionand of an electron discharge device of the conventional type.

Referring to the drawing, the electron discharge device shown in Figure1 has a dome shaped evacuated envelope 9 with the usual reentrant stemand press l9 and the usual base H. The envelope or bulb 9 encloses aunipotential i cylindrical cathode I 2 having a heater IS, a tubularhelical grid electrode 14 supported from the press It on grid side rodsand I6; and a tubular anode or plate electrode H, with side flanges I8to which the plate support rods 9 are attached, the grid and anode beingcoaxial with and surrounding the cathode. The anode carries from theflanges [8 the upper mica spacer 20 by the metal straps 2i and the lowermica spacer 22 by similar metal straps 23.

In the tube shown in Figure l the helical grid 14 is, as best shown inFigure 2, elliptical in cross section, the anode I! being of theconventional type and circular in cross section; in the modificationshown in Figure 3 the grid 24 is made like the helical grid [4, but iscircular in cross section,

pendicular to each other.

In these embodiments of my invention the spacing between the grid andanode is at a minimum at the grid side rods, and each constructionillustrated produces a tube having improved characteristics,particularly an amplification factor which is substantially 1 constantover a wide range of grid bias voltages.

The preferred explanation of the improved results observed in tubesembodying my invention can best be followed in connection with Figures 5and 6, which are a graphical comparison of an electron discharge devicemade in accordance with my invention with an elliptical grid and acircular plate, and indicated by diagram A, with the usual electrondischarge device with a circular grid and a circular plate and indicatedby diagram B. Both diagrams represent a cross section of a tube having astraight cylindrical cathode I2, a tubular plate I! circular in crossThe fiow of the electron stream from the cathode to the plate ishindered to some extent by the grid side rod and the electrostatic fieldaround it, hence that part of the plate behind or in the electron shadowof the grid side rod re ceives less current than the other parts.

Figures 5 and 6 the dotted line circles concentric with the side rods l5and marked low voltage represent the eiiective fields around the siderods when a low negative voltage is impressed on the grid. That part ofthe plate behind the grid side rod is in the electron shadow of thisfield,

which hinders the flow of electrons from the cathode, and in effectreduces the effective emitting surface of the cathode.

The extent of this shadow is represented in the diagrams by the portionof the anode II which lies between the dotted lines extending from thecathode to the plate and tangent to the circles representing theeffective fields around the grid side rods.

If the rid bias is increased so that its efiective field is increased,as represented by the dotted line circles concentric with the side rodsIS, the shadow on the plate is materially increased, as graphicallyshown by the dotted lines tangent to these dotted line circles.

.As the grid bias or impressed, negative voltage is increased the shadoweffect is materially increased and the effective emitting surface of thecathode reduced.

The shadow effect causes the amplification factor or mu of the tube tobe different for different impressed grid voltages. In the usual tubehaving a circular grid and plate, the mu for the various sectors of thegrid is different, being greatest at the side rods, as the large siderods have a greater controlling effect on the electron stream than thesmall grid wire. The efiective amplification factor of the tube is anaverage of the factors for the various sectors. As the grid is made morenegative the "shadow effect is increased and the effective surface ofthe cathode reduced until at some negative bias the emission from thoseparts of the cathode adjacent the grid rods cannot reach the plate andis practically cut off. Whereupon those portions of the grid causingthis cut-off are no longer effective in controlling variations in theelectron flow to the plate. The portions of the grid which first reachthe cut-off stage are the side rods and the parts nearest the side rods,and since the mu is the greatest at these portions of the grid, theeffective amplification factor of the grid is made to depend on thelower mu sections and the effectlve amplification of the tube istherefore decreased. The higher the impressed negative voltage, thegreater the shadow effect and the lower the effective amplificationfactor of the tube. This characteristic is particularly objectionable inan amplifier, such as an audio amplifier designed for constant mu tubes,since the amplifier is depended upon to reproduce faithfully in itsoutput circuit the variations in voltage applied to the grid of the tubein the input circuit. If the more negative impressed grid voltages areamplified to a lesser extent than the less negative impressed gridvoltages, as is done in the conventional tube, distortion in the outputcircuit of the tube will result, hence the grid swings in the usual tubehaving a circular anode and grid, must be limited to a narrow range toavoid this distortion.

My invention reduces this undesirable shadow effect to a minimum andmakes the amplification factor more nearly constant, as appears fromFigure 5 which shows that with the elliptical grid and circular platethe shadow effect caused by the effective field around the grid siderods is not only materially less for both low and high bias than with acircular grid and circular plate, but also that the shadow effect doesnot change as much with changes in the bias of the grid. In a tubeembodying my invention the side rods I5 and I6 are spaced at the minimumdistance from the plate I1, and do not exert as much retarding effect onthe electrons leaving the cathode as the side rods of the usual circulargrid. As those portions of my grid which correspond to the higher musectors of the conventional circular grid are closer to the plate thanthe sectors corresponding to the lower mu sectors, the various sectorshave substantially the same mu, and also due to the resulting largereffective emitting area of the cathode, the mutual conductance of thetube is increased.

In accordance with my invention the plate may be circular in crosssection and the grid elliptical, or the plate elliptical and the gridcircular, or both elliptical with their major axis perpendicular, but inall cases the spacing between the plate and the grid electrode is a atthe grid side rods.

An elliptical type grid permits the greatest separation between thecathode and side rods for a given transconductance. Placing the gridside rods further away from the cathode decreases the chance of shortcircuits between the grid and the cathode, decreases grid emission, asthe grid heating is less, and permits the distance between the grid rcdholes in the mica spacers to be increased, whereby the spacers arestronger than in the usual type of construction.

My invention is of considerable utility in multigrid tubes, such asspace-charge grid tubes and similar tubes in which the grid nearest theoathode is at a positive potential with respect to the cathode. The siderods of such positive grids of the round type draw a high current whichis out of proportion to the value as a control section of the grid. Theside rods of an elliptical spacecharge grid are much further away fromthe cathode, have less effect, and draw appreciably less current withthe result that the amplification factor is more nearly constant.

In addition to the advantages of the elliptical grid structure for thegrid nearest the cathode, it is found that this structure is also ofadvantage for other grids in the tube. It permits the side rods of thevarious grids to be separated sufliciently to meet the requirements ofgood mechanical design, and at the same time permits the wires of thedifferent grids to be as close together as is desirable for electricalreasons for the best operation of the various grids.

While I have indicated the preferred embodiments of my invention ofwhich I am now aware and have also indicated only one specificapplication for which my invention may be employed, it will be apparentthat my invention is by no means limited to the exact forms illustratedor the use indicated, but that many variations may be made in theparticular structure used and the purpose for which it is employedwithout departing from the scope of my invention as set forth in theappended claims.

What I claim as new is:

1. An electron discharge device having a straight cylindrical cathode, atubular grid electrode surrounding and coaxial with said cathode andhaving side rods parallel to said cathode, a tubular plate electrodesurrounding said grid electrode and coaxial with said cathode, saidplate electrode being elliptical in cross section, the spacing betweensaid electrodes being at a minimum at said side rods.

2. An electron discharge device having a straight cylindrical cathode, atubular grid electrode surrounding and coaxial with said cathode andhaving side rods parallel to said cathode, a tubular electrodesurrounding said grid electrode and co-axial with said cathode, saidgrid electrode being circular in cross section and said tubularelectrode surrounding said grid electrode being elliptical in crosssection, the spacing between said electrodes being at a minimum at saidside rods.

3. An electron discharge device having a straight cathode circular incross section, a tubular grid electrode surrounding and coaxial withsaid cathode and having side rods parallel to said cathode, a tubularplate electrode surrounding said grid electrode and coaxial with saidcathode, said electrodes being elliptical in cross-section and mountedwith their major axes perpendicular to each other, said grid side rodslying in a plane passing through the major axis of said 7 grid.

BERNARD SALZBERG.

